1. Field of the Invention
The invention relates to systems for imaging a fog obscured scene such as a body of water (e.g., a harbor) or a runway from a landing aircraft, particularly with respect to optical image processing radar systems. The invention is useful not only in fog obscured environments but also with respect to other atmospheric obscurants such as haze, smoke, rain, snow, dust, and the like. The invention also relates to determining the range to illuminated targets in the scene.
2. Description of the Prior Art
A present day need exists for an inexpensive, reliable, compact and lightweight system for visually observing a scene in an optically obscured or attenuating environment. Atmospheric obscurants such as fog, rain, snow, smoke, dust and haze may render targets within the range of the system unobservable. Visual underwater observation systems also suffer from operating in a highly attenuating and obscuring optical environment. In, for example, laser illuminated television viewing systems, light backscattered from the interfering atmospheric entities obscures the viewing of forward illuminated targets. The backscattered light results from reflections off the atmospheric obscurants between the observer and a target of interest and from the target to the further distances. This non-imaging light, which is familiar to one who has driven in fog, obscures the desired imaging light in a background of optical noise.
Thus, it is appreciated that light directed from the observer to a target is scattered. Scattering changes the direction of a portion of the light beam and the thicker the atmosphere the more scattering that occurs. The backscattered light is reflected directly back at the observer, e.g., headlights in fog, obscuring the desired optical signal in a sea of optical noise.
Imaging systems for seeing through fog have been attempted utilizing blackbody radiation in the far infrared region of the electromagnetic spectrum. Although such systems have been utilized in aircraft, they require cryogenic cooling and very large lenses due to the long wavelengths of the thermal radiation. Hence, such systems tend to be very expensive, heavy and bulky and require excessive maintenance.
Additionally, the infrared camera is generally fixed relative to the aircraft and viewing is accomplished by observing a cathode ray tube display. Thus, the pilot, when using such a system, is not looking in the direction of interest such as out of the windshield and to the left. A helmet mounted display could, however, be utilized which servoes the camera to follow the head movement of the pilot. Such arrangements are complicated, expensive, heavy and bulky and require moving parts which tend to be unreliable.
Presently, the only other landing aids available are radar beacons and voice contact with air traffic controllers who are observing the position of the aircraft via ground based radar. In present day aircraft pilotage, it is essential that at some point of the approach, the pilot must either establish visual contact with the runway or abort the landing. Thus, landing an aircraft in very poor visibility can be extremely dangerous. Abortion of a landing may not be feasible because of such factors as low fuel, fog obscured nearly airports, and mechanical or medical emergencies. In a military environment, the aircraft may be damaged rendering landing abortion impossible. Thus, a desideratum of present day aircraft pilotage would be to render a runway visible to a pilot, which runway would otherwise be blind. Any improvement over the poor visibility situation could make the difference between a successful landing and a crash.
The problems discussed above are also prevalent in closed circuit television systems for observing a body of water such as a harbor or an estuary. Surveillance may be desirable both above and under the surface. Presently, there is no known apparatus for clearing atmospheric obscurants such as fog, haze, smoke and the like from a closed circuit television or other optical system field to view. Such apparatus would be highly desirable to the pilot of an aircraft, driver of a car or captain of a ship to see through a low visibility atmospheres such a described above in a fast, real time manner.